Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
26
Analysis of post-blasting source mechanisms of mining-
induced seismic events in Rudna copper mine, Poland
Alicja Caputa1, Adam Talaga2, Łukasz Rudziński3
1Department of Applied Geology, Faculty of Earth Sciences, University of Silesia, 60 Bedzinska Str, 41-200
Sosnowiec, Poland;
corresponding author: [email protected] 2KGHM Polska Miedź S.A., 48 M. Skłodowskiej-Curie Str., 59-301 Lubin, Poland;, [email protected] 3Institute of Geophysics PAS, Department of Seismology, Ksiecia Janusza 64 str. 01 - 452 Warszawa, Poland;
Received: 28th May, 2015
Accepted: 11th November, 2015
Abstract
The exploitation of georesources by underground mining can be responsible for seismic activity in areas
considered aseismic. Since strong seismic events are connected with rockburst hazard, it is a continuous
requirement to reduce seismic risk. One of the most effective methods to do so is blasting in potentially hazardous
mining panels. In this way, small to moderate tremors are provoked and stress accumulation is substantially
reduced. In this paper we present an analysis of post-blasting events using Full Moment Tensor (MT) inversion at
the Rudna mine, Poland, underground seismic network. In addition, we describe the problems we faced when
analyzing seismic signals. Our studies show that focal mechanisms for events that occurred after blasts exhibit
common features in the MT solution. The strong isotropic and small Double Couple (DC) component of the MT,
indicate that these events were provoked by detonations. On the other hand, post-blasting MT is considerably
different than the MT obtained for strong mining events. We believe that seismological analysis of provoked and
unprovoked events can be a very useful tool in confirming the effectiveness of blasting in seismic hazard reduction
in mining areas.
Key words: source mechanism, blasting, foci
Introduction
Underground exploitation of copper ore in deep
copper mines in the Lower Silesian Copper
District, Poland (LSCD) is associated with
many mining hazards. In Polish copper mines
undoubtedly the most significant danger is high
seismic activity. The strongest mining tremors
can be considered as small earthquakes
associated with rockburst, and this is a
continuous problem during exploitation. The
rockburst hazard is caused by high-energy
tremors induced by mining operations.
Historically, the first significant mining-
induced event (M 2.8) occurred on 31 July 1972
at the Lubin Mine, one of the three deep copper
mines located in the LSCD. Since that time, the
continued progress of ore extraction has
produced a regular increase in the number and
strength of recorded events (Butra, 2011). Large
high-energy seismic events, (E>105J; 11,200
events during 1990-2010), with the cumulative
energy of 70,48 GJ and 323 events between
1990 and 2010 (Butra, 2011), indicate that the
seismic hazard in Lower Silesian copper mines
cannot be ignored. To decrease such hazardous
situations, mining management utilize several
preventative measures, including technical,
active and organizational methods. The most
effective approach is by utilizing a group of
Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
27
active methods, consisting of provoking
tremors by the detonation of an explosive
charge in the blast holes. This is especially
important in mining panels with a considerably
high level of rockburst hazard.
Since mining extraction based on both
room-and-pillars and longwall methods can
produce seismic activity by changes in the local
stress field (Gibowicz and Kijko, 1994), active
prevention with properly designed blasting can
be a very effective method to reduce the risk for
miners and underground infrastructure. In this
paper, using focal mechanisms analysis based
on Full Moment Tensor (MT) inversion, we
show that blasting is readily utilized to produce
controlled seismic events. Moreover, events
after blasts are characterized by a quite strong
MT isotropic component, which contradicts MT
obtained for strong seismic events recorded by
Rudna’s mine seismic network.
Characterization of the Lower Silesian
Copper District
The Lower Silesian Copper District is located in
SW Poland. Currently, the district is one of the
most important mining areas in Europe,
producing 30.2 million tons of copper ore per
year (www.kghm.pl). In this area, three
underground mines (Fig. 1), “Lubin”,
“Polkowice-Sieroszowice” and “Rudna”, are in
operation (Butra, 2010). Rich copper deposits
were discovered at the end of the 1950s, and ore
exploitation started in 1967 at the Lubin mine.
The stratoidal type copper deposit on the Fore-
sudetic Monocline is characterized by a small
inclination (approximately 4°), variable
thickness (from 0.4 to 26 m), and a varying
lithological profile - sandstones, dolomites and
ore shales can be found at depths between 600
m and 1400 m below the surface.
The analysis presented in this paper is based
on data recorded from the Rudna mine
underground seismic network. The network
consists of 32 vertical Willmore II and III
seismometers (1-100 Hz). Signals are recorded
with a sampling rate of 500 Hz and dynamic
range of less than 66 dB (Koziarz and Szłapka
2008). The mine operates at depths of 950 m to
1150 m with a room and pillar (pillar and stall)
exploitation system. The copper ore is
excavated with rooms (bored by explosives)
located perpendicularly to the exploitation front
line and stripes located parallel, leaving pillars
supporting the roof over excavated areas
(Lizurek et al., 2015). To extract the output, the
detonation of explosive charges is used in all
Polish copper mines.
Determination of the focal mechanism of
mining tremors
The focal mechanism of mining seismic events,
such as for natural earthquakes, can be
described by certain systems of couples forces
that act on a particular piece of rock medium.
As a result of this action, a tremor focus is
initiated (Dubiński, 2013). The main goal of
determining the focal mechanism is to define
the force distribution in the source, which leads
to the movement of the elastic energy emission
and finally, to seismic wave propagation.
Assuming that the source can be described by a
pure Double Couple (DC), the mechanism of
the seismic event can be considered as spatial
orientations of two, perpendicular nodal planes,
separating the two areas of compression and
dilatation inside the source. While one of these
planes is the actual focal plane on which the
movement took place and the emission wave
started, the second is considered as an auxiliary
plane (Dubiński, 2013) (Fig. 2). The spatial
location of these planes is described by three
angles φ(strike), δ(dip), λ(slip angle).
Focal mechanism analysis in Polish mining
seismology has been carried out since the
beginning of the 1990s (Wiejacz 1991,
Gibowicz & Kijko, 1994) and nowadays is one
of the most important methods employed to
investigate and understand mining induced
seismicity.
Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
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Fig.1. Location of LSCD with three deep copper mines: Lubin, Polkowice - Sieroszowice and Rudna.
Fig.2. The P- wave radiation pattern for a double couple source (Stein & Wysession, 2003)
Since that time, based on digital seismograms, a
MT inversion rather than a pure DC solution, is
a more appropriate way to estimate the source
mechanism of mining events in Polish mines.
This method is based on seismogram analyses
and assumes that the displacement recorded in
the far field (uk) is caused by a system of forces
acting on the focus, and is the sum of the
displacements caused by the particular force
couples (Aki & Richards 1980). This movement
can be described by the equation:
𝑢𝑘 =𝑀𝑖𝑗∗𝜕𝐺𝑘𝑖
𝜕𝑥𝑗= 𝑀𝑖𝑗 ∗ 𝐺𝑘𝑖,𝑗 (1)
where Mij (Full Moment Tensor) describes the
moment of the force couples acting in the
direction of the xi axis of the arm in line with
the xj axis; Gij (Green’s function), describes the
impulse response of the medium for the distance
traveled by the seismic wave; * – convolution
operation.
The MT fully describes the system of forces
that are acting in the seismic source, which must
be assumed to have a point nature. This
Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
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assumption is fulfilled as the distances between
the hypocenter and receivers are much longer
than the size of the source. The MT as a physical
factor is a linear combination of force couples
and the moments. Another assumption in this
method is that all of the seismic moment tensor
components depend on time in the same way,
meaning there is a synchronous source
generating the same changes in time in all
directions (Stec, 2009). Applying these two
statements, the displacement field equation (1)
can be written as:
𝑢𝑘(𝑥, 𝑡) = 𝑀𝑖𝑗[𝐺𝑘𝑖,𝑗 ∗ 𝑠(𝑡)] (2)
where s(t), the Source Time Function (STF),
characterizes changes of a seismic source in
time.
As follows from equation (2) the
displacement field uk is a linear function of the
components of the seismic moment tensor and
the terms in square brackets. With the
assumption that STF is described by the Dirac
delta (Stec, 2009), the displacement field uk can
be written as:
𝑢𝑘 = 𝑀𝑖𝑗𝐺𝑘𝑖,𝑗 (3)
The MT (Mij in equation above) can be
presented as a nine-component matrix M with
dimensions of 3x3:
𝑀 = [
𝑀11 𝑀12 𝑀13
𝑀21 𝑀22 𝑀23
𝑀31 𝑀32 𝑀33
] (4)
The moment tensor provides a general
representation of the internally generated forces
that can act at a point in an elastic medium. The
rule that angular momentum has to be
conserved requires that M is a symmetric tensor
and has only six independent elements (Shearer,
2009). Every component of the MT matrix
represents another pair of forces acting in the
source of the event (Fig. 3).
Fig.3. The nine different force couples that make up
the components of the moment tensor. (Aki &
Richards, 1980)
Such a defined MT can be further
decomposed in other ways. The most acceptable
in mining seismology is decomposition into the
isotropic component (M0) and the deviatoric
component (M’), which is written as follows
(Jost and Hermann, 1989):
𝑀 = 𝑀0 +𝑀′ (5)
In M0, the diagonal components (where i = j)
describe the force couples within the moment,
which are directed along the main axis. They are
responsible for the volume changes in the
tremor source and are the evidence of the
explosion (when the value is positive /+/) or of
the implosion (if the value is negative /-/).
The deviatoric component of MT (M’) can
be further decomposed, although with some
inaccuracy, into a Compensated Linear Vector
Dipole (CLVD) (corresponding to uniaxial
compression (/-/) or tension (/+/) and double
force couples (DC) (indicating a pure shear
motion in the event source, i.e. on the fault
plane). Decomposition of the deviatoric
component can be written as:
Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
30
𝑀′ = 𝑀𝐷𝐶 +𝑀𝐶𝐿𝑉𝐷 (6)
Therefore, the full decomposition of the MT
(Fig. 4) is defined as:
𝑀 = 𝑀0 +𝑀𝐶𝐿𝑉𝐷 +𝑀𝐷𝐶 (7)
The procedure to find the MT (Moment
Tenor Inversion) is based on seismic signals
recorded at several seismic stations,
surrounding the source. Since MT has 6
independent elements, at least seven signals
should be available. Utilizing the correct
procedure the inversion is optimized, i.e. the
minimization of the variation between
synthetics and observations in a chosen norm
(Gibowicz and Kijko 1994).
Therefore, based on seismogram analysis
and further on the decomposition of the seismic
moment tensor, we can estimate a point source
mechanism of the seismic event. The
interpretation of the explosive or implosive
point source model (i.e. high isotropic
component) corresponds to the process of the
volume changes within the rock mass. One of
the reasons for this could be shooting (blast) at
the selected layer of the deposit or in its close
vicinity. Another reason could be rock
destruction by the pressure of a large overlying
rock mass. The CLVD indicates uniaxial
compression or tension and may involve the
destruction of pillars. Finally, the model
represents the mechanism described by a DC
component corresponding with events
associated with the cracking of thick and dense
strata with a high degree of stiffness and
strength, or the movement of their fragments.
Commonly, it refers to undermined complexes
of roof rocks. The DC component is generally
highest in the case of natural and strong events
and is more relevant when describing tectonic
earthquakes. Since, in case of mining
seismology, the strong non-DC components
play a significant role, we are interested in the
full MT rather than just the DC solution.
The Rudna Mine case study
Site and data description
All events used in our analysis occurred on the
G-11/8 mining panel (Fig. 5) between January
2012 and November 2014. During this time
more than 15,000 induced seismic events were
recorded but only 135 were strong enough
(E>105 J) to for MT analysis.
A cluster of post-blasting events comprised
of tremors took place at the moment of
explosive charge detonation (classified as event
0 seconds after blasting by Rudna Geophysical
Mining Survey), and immediately after blasting
(21 seconds after detonation). The second part
of this group was created by two tremors during
the post-blast waiting time, respectively in 5
minutes and 4 hours and 7 minutes after
blasting.
Fig.4. Graphic interpretation of the seismic moment tensor inversion (after Talaga, 2014)
Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
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Fig.5. Epicenters (blue stars) mark the localization
of analyzed events within mining panel G-11/8 at
Rudna copper mine. The seismic network is
represented by red triangles
From this group we selected 4 events, which
occurred just after blasting and compared their
mechanisms with the strong, mining-induced
seismic tremors that occurred on the same panel
in the vicinity of the selected cluster.
MT inversion: P – wave amplitude
All analyses and calculations during this study
were performed using FOCI software (Kwiatek,
2013), with the MT inversion in the time
domain from amplitudes of P-wave onset
recorded by the underground seismic network at
Rudna Mine (Fig. 5). The velocity model of the
Rudna mine assumes that the recorded first
onsets maybe of different types (Król 1998):
direct P-waves, which are observed on
seismograms recorded closer than 1 km from
the hypocenter, and two refracted waves. The
refracted P-waves in LSCD’s conditions are
divided into refractive wave A and refractive
wave B. The A-wave is defined as a seismic
wave refracted from the deposit overlying an
anhydrite layer (Lizurek et al., 2015) and is
recorded between 1 km to approximately
2.8 km from the hypocenter. The B-wave is a
wave refracted from the thick strata of
sandstone underlying the shale layer (Lizurek et
al., 2015) and is recorded by seismometers
located at a distance of more than 2.8 km from
the source. The velocities of wave propagation
were specified based on seismological
observations: 5 km/s for the direct wave, 5.9
km/s for the refracted A-wave, and 5.6 km/s for
the refracted B-wave (Król, 1998).
In the present study all types of waves were
used, with 32 onsets at 32 stations located in the
Rudna mine area. The input parameters are the
amplitude and polarity information on the first
P-wave displacement pulses. According to
Fitch et al. (1980), the recorded displacement
for the vertical component of the P-wave phase
is (Lizurek et al., 2015):
uzP(x, t) =
1
4πρα3r[γMs (t-
r
α) γ] lz (8)
where ρ is the average density, r is the source-
receiver distance, α is the average velocity of P-
wave, M is the seismic moment, lz is the cosine
of the angle of the incidence, and γ is the take-
off angle, s is the STF.
Such a procedure is part of the FOCI
software and allows for the calculation of the
full MT as well as pure DC solution.
During this analysis we used the full
moment tensor inversion using L2 norm. The
MT was decomposed into the isotropic
component (volume change), CLVD (linear
compression/dilatation) and DC (shear motion).
The former, namely the volume change
component of the solution, is key for this work
as it can show that there was an increase of
volume in the source, and can be treated as an
indicator that a blasting mechanism was the
reason for the event. In general we expect, in
this case, that the DC component after the
explosion is low.
The principal problems of determining source
mechanism
All geophysical analyses are exposed to
measurement errors and many factors that
Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
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reduce the quality of results should be taken into
account. The principal issues in determining the
source mechanism of induced tremors using
FOCI software were:
- large errors of the Z (depth) coordinate
estimation – since FOCI provides tremor depth
tests, in many cases it was necessary to change
and correct the depth of the hypocenter based on
the MT error (Fig. 6);
- in the case of small events, low wave
amplitudes cause difficulties in sorting first
arrivals (Fig. 7);
- saturated records and consequently clipped
signals from the high energetic seismic event
(Fig. 8);
- resonance signal (Fig. 9) - no information of
P-wave arrivals.
- electrical disturbances (Fig. 10)
- no signal in selected channels
- filter artifacts, which could be apparently
treated as a P-wave onset (Fig 11);
Fig.6. The sample chart of tensor error generated in FOCI software during depths test.
Fig.7. Low amplitudes of P-wave first arrival presented on a seismogram for small event (E=1*105J) recorded at
a station 2.6 km away from the hypocenter.
Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
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Fig.8. Clipped signal presented as a seismogram 1.4 km away from hypocenter.
Fig.9. Resonance signal presented as a seismogram
Fig.10. Electrical disturbances presented as a seismogram
Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
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Fig.11. P-wave onset marking and filter artifact (red arrow) presented as a seismogram
Results of source mechanism calculations for
selected tremors
In Table 1 presents the MT for the event
occurring at the time of detonation. There was
group winning blasting combined with release
blasting in the roof of the mining level. For this
operation 3582 kg in total of explosive charge
was used (1726 kg rock solid, plus 1856 kg in
roof). The most characteristic issue is a high
percentage of explosive components showing
volume changes in a source. When the value is
positive it can be considered as compression
due to blasting in the source. In this case, the
highest value of the decomposed elements is
found in the linear dilatation (CLVD) and is
equal to almost 60 %. Linear extension (L1) in
the foci is influenced by an explosion at the
source. Taking into account that the highest
effects of the event occurred in the roof of the
deposit level, the extension may be explained by
the redistribution of stress near the excavation
area. Finally, the small DC component of the
MT indicates, that shearing motion on nodal
planes can be negligible. This statement is in
agreement with our expectation that seismic
events controlled by blasting exhibit strong
non-DC focal mechanisms.
Similar results and explanations are also
seen for the next two examples of post-blasting
events. In the case of the second example (Table
2) tremors were induced by group winning
blasting, where 2131 kg of explosive charge
was fired.
Tab.1. Source mechanism (full MT and
decomposition) of tremor in 0 s after blasting.
Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
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Tab.2. Source mechanism of tremor in 21 s after
blasting.
A smaller isotropic component than in first case
can be directly connected to a longer time
interval between the blast and the origin time of
the event. Nevertheless, both high CLVD and
very small DC partially support idea that this
event was provoked by detonations, although
was not observed during blasting. The analyzed
event in Table 3 pertains to the post-blast
waiting-time tremor (i.e. occurred at the time
when seismic events are expected). This event
was controlled by a group winning blasting,
where 2136 kg of explosive charge was
detonated. Components of the MT in this case
are quite similar to the two previous solutions.
The isotropic component is still quite high and
its positive value indicates an increase in
volume in the source. As in previous results
linear extension has the highest value. The DC
value is the only contrast. In this case the DC
component presents the higher value of the full
moment tensor solution. This can be explained
by a larger share of shear motion in the source
due to the partial relaxation of the post blasting
pressure, and the release of cumulated energy
on the fault plane. characterized by the highest
DC component of all previously analyzed
examples.
Tab.3. Source mechanism of tremor in 5 min after
blasting
Group winning blasting, utilizing 2103 kg of
explosive charge, provoked the source
mechanism of post-blasting tremors, presented
in Table 4. This MT solution is characterized by
the highest DC component of all previously
analyzed examples. The seismic moment tensor
decomposition indicates that there was a more
significant share of shear motion (32 %) and
Contemp.Trends.Geosci., 4(1),2015,26-38 DOI:10.1515/ctg-2015-0003
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linear extension (48 %) at the event source. This
DC component value and the much higher
energy (106J) of the tremor could be an indicator
of the elastic rebound of the rock mass.
Nevertheless, a relatively high positive value
for the isotropic component suggests an
explosive origin for this event. Although this
particular event occurred more than four hours
after detonation, it was still during the post-blast
waiting time. Full MT solution for this event
also supports idea that blasting successfully
provoked it.
Tab.4. Source mechanism of tremor in 4 h 7 min
after blasting
The final seismic event analyzed was the
strongest, and occurred in the panel G-11/8
during the period of observation 2012-2014.
The event was classified as natural (i.e. did not
occur during waiting time after blasting). The
full MT for this example is presented in Table
5. There are visible differences between this
focal mechanism and the solutions of post-
blasting tremors. Isotropic components in the
last MT solution indicates compression forces
in the source, which is completely in
contradiction with previous solutions. The
value of the compensated linear vector dipole is
also evidence of linear compression. Thus,
isotropic and CLVD components are related to
a compressive stress field close to the
excavation level (acting especially in the pillars
and undisturbed part of deposit).
Tab.5. Source mechanism of mining induced tremor
unrelated with blasting
A high value for the DC component may
indicate significant shear motion on a
discontinuity or on the fault plane of a
preexisting fault. The distribution of MT
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component values in this event, especially a
high DC component, can be an indicator of
"typical" mining induced tremors. Such types of
events usually appear spontaneously and can
pose a serious threat to miners.
6. Discussion and conclusions
Moment Tensor inversion is potentially a
very useful tool to determine a point source
mechanism. Although in global seismology this
approach is commonly used during
seismological investigations, in the case of
Polish mining seismology it is still not a popular
procedure. To obtain reasonable results some
factors need to be considered, among other
problems with seismic signal recordings
described in this paper. MT analysis shows that
post-blasting, moderate mining tremors are
characterized by very interesting features.
Isotropic components of MTs for this group of
events, as well as small DC components, could
be the main factors differentiating post-blasting
seismicity from other spontaneous events. This
is also supported by MT of strong-induced
tremors. Also worth noting is the growing value
of the double couple component followed by an
increase in time between detonations and event
origins. Increase of the DC component with
longer post-blast waiting times can be evidence
of stress distribution changes and the elastic
rebound of the rock mass We can interpret such
kind of events as partially acting on preexisting
weak geological zones. Although detonation
did not cause immediate tremors, changes in
stress fields were strong enough to provoke
them some time afterwards. This is a real
concern in mine blast prevention. This
interpretation is also supported by a decrease in
isotropic components of the events, which
correspond well to the activation of weak
geological zones within the rock mass. We
noted that the highest isotropic component was
estimated for events provoked with more than
3500 kg of explosive material. On the other
hand, however, for the rest of the provoked
events MTs were characterized with similar
features. We believe that the amount of
explosives did not influence our general
conclusions; nevertheless, additional influence
of the weight of explosives cannot be excluded.
Results presented in this paper show strongly a
potential for the use of more sophisticated
seismological analyses to confirm that active
prevention can reduce seismic risk on mining
panels with high rockburst hazard.
Acknowledgements
The Reviewers are acknowledged for their
effort and help in improving the paper. Łukasz
Rudziński was partially supported within
statutory activities No. 3841/E-41/S/2015 of the
Ministry of Sciences and Higher Education of
Poland.
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